Summary

Knowledge of root and canal morphology is a prerequisite for successful endodontic treatment. The external and internal morphological features of roots are variable and complex. Current advancements in non-destructive digital image systems, such as cone-beam computed tomography (CBCT) and micro-computed tomography (micro-CT), allow detailed qualitative and quantitative analyses of root and canal morphology. This growing body of knowledge has paved the way for revising several historical concepts and introducing new perspectives for more accurate descriptions of root and canal morphology in teaching, research, and clinical practice. This chapter aims to provide an update on the application of a new system for classifying root and canal morphology, accessory canals, and anomalies, to discuss anatomy of the root apex and apical foramen, and to present the growing body of knowledge on root and canal morphology in all tooth types.

1.1 Introduction

Effective endodontic treatment requires a thorough knowledge and understanding of root and canal anatomy [1]. For decades, this topic has been the subject of many experimental and clinical reports using a wide variety of techniques such as injection of vulcanized rubber following by decalcification, staining and clearing, 2D radiographic imaging, scanning electron microscopy, cone beam computed tomography (CBCT) and micro-computed tomography (micro-CT) [2–7] (Figures 1.1 and 1.2), and it is obvious that root and canal morphology varies greatly between different tooth types and populations, within populations, and even within the same individual [1, 7–9]. Recently, new perspectives on the characterisation of root canal configurations, accessory canals, and anomalies [10–13] have emerged, in addition to growing knowledge related to the fine details of the apical canal morphology [14, 15], and anatomical variations amongst different population groups [6, 16–18]. In this chapter, the application of a new coding system for classifying root and canal anatomy is described, and recent advances on the morphology of the root apex and apical foramen are discussed. In addition, this chapter presents updated information on root and canal anatomy evidenced in prevalence studies using CBCT technology.

1.2 Different Perspectives in Characterizing Root and Canal Morphology

Classifications play a central role in science, where they are used not only as a way to organise knowledge but also as a powerful tool for accurately defining characteristic features of a given subject [19]. Data generated from the classical work of Hess and Zurcher [2] to the more recent studies demonstrate that the ever-expanding knowledge on this subject required the creation of a classification system for defining root canal configurations. The Vertucci classification [3] is the most commonly used system for categorising canal morphology; however, several reports identified considerable deficiencies in this system [10, 13]. In this section, these deficiencies are discussed and a new coding system for classifying root canal morphology, accessory canals, and anomalies is described.

1.2.1 Deficiencies of Current Classification Systems for Root Canal Morphology

Using sectioning and 2D radiographic methods, Weine et al. [4, 20] categorised root canal configurations within a single root into four types depending on the pattern of division of the main root canal along its course from the pulp chamber to the root apex (Figure 1.3a). Later, Vertucci et al. [21] developed a classification system based on the evaluation of 200 cleared maxillary second premolars, and identified a total of eight configurations (Figure 1.3b). Investigators [22–25] added 15 more supplemental canal configurations to Vertucci’s classification (Figure 1.3c) with other non-classifiable configuration types being recently introduced [26].

The systems proposed by Vertucci et al. [21] (and its supplemental categories) have been the most commonly used and have been beneficial when categorising many canal configurations. However, considerable deficiencies exist because of the following:

1.2.1.1 No Consideration of the Number of Roots in Anterior and Premolar Teeth

In Vertucci’s classification, there is no description of the number of roots in anterior and premolar teeth. Therefore, it is not possible to differentiate between Vertucci type IV (2 separate root canals) in a single- or a double-rooted tooth (Figure 1.4a, b). Similarly, it is not possible to define Vertucci type V (1-2) that can exist in a single- or a double-rooted tooth (Figure 1.4c, d). Clearly, in terms of the clinical management of teeth undergoing root canal treatment and endodontic surgery, it is critical to define the number of roots and not just canals, as this will have implications in terms of access cavity preparation as well as visualisation of canal orifices, instrumentation, post preparation, and root-end resection, if indicated [8]. In addition, all three-canalled maxillary (or mandibular) premolars (or anteriors) are coded as Vertucci type VIII with no consideration given to the number of roots or level of the canal bifurcations (Figure 1.4e, f).

To overcome this deficiency in Vertucci’s classification, the number of roots has been described in various case reports and studies alongside the Vertucci classification but with no details on their location, and this is also considered insufficient. For instance, double-rooted anterior teeth can exist in two forms (mesial and distal, or buccal and palatal/lingual) [27]. Three-rooted maxillary premolars can have two forms (i.e. two buccal roots and one palatal root or one buccal root and two palatal roots) [8, 10, 28]. Therefore, it is not only important to present the number of roots of a given tooth but also to describe the location of these roots as this detailed description of the roots has clinical implications at different phases of treatment.

1.2.1.2 Absence of Clear Definitions of Root Canal Components

In Vertucci’s classification, the root canal orifice is defined as ‘a root canal begins as a funnel-shaped canal orifice generally present at or slightly apical to the cervical line,’ [1], with no description of this ‘slight apical’ position. One possible reason is that the Vertucci classification was based on staining and clearing methods in which teeth were subject to decalcification, staining, and clearing; such procedures significantly deteriorate the normal anatomical features of the tooth, including the cemento-enamel junction (CEJ), thus making its identification, in some samples, rather challenging. Notably, in multi-rooted teeth, the location of the pulp chamber floor may not coincide with the CEJ [1, 29].

Another potential confusion exists over how to define inter-canal communications, which can be classified as an integral part of the root canal configuration with an impact on its classification, or simply as a minor feature with no impact on its anatomical classification (Figure 1.5). An inter-canal communication (transverse canal anastomosis, canal isthmus) has been defined by the American Association of Endodontists (AAE) [30] as a thin communication between two or more canals in the same root or between vascular elements in tissues. Classifying the root canals using the Vertucci classification could vary and become more complicated if inter-canal communications are considered as a part of the main canal configuration (Figure 1.5). Since the criteria for defining inter-canal communications were not mentioned [3] (Figure 1.5), the confusion is more obvious when micro-CT studies continue to report many canal configurations as ‘non-classifiable’ when using the Vertucci classification [31, 32]. This may well be the case for some ‘complicated’ canal configurations but is misleading for many other types because such studies have always included transverse canal anastomosis as a part of the main canal configuration [31, 32]. At the same time, several CBCT studies either have not considered transverse canals as a part of the root canal configuration or did not mention the criteria of transverse canals if they are not meant to be considered [6, 33–35]. As a consequence of this variation in interpretation, comparison amongst studies creates conflicts not only because of the different methods used to prepare the specimens but also because the same classification system is being used in a different manner.

The identification of a transverse canal anastomosis separately from the canal configuration is a concern because they have clinical implications during chemo-mechanical instrumentation, canal filling, and root-end cavity preparation and filling [36–38]. In addition, transverse canal anastomosis may communicate with the external root surface and be a pathway for microorganisms and their associated toxins into the lateral periodontal and periapical tissues, thus affecting clinical outcomes [39].

In addition, there is confusion with regards to apical canal bifurcations – when it is a part of the configuration and when it is considered as an accessory canal. Similar confusion exists for apical root bifurcations whether a tooth with a bifid/small double root apex is considered as a single- or double-rooted tooth, which is discussed later in this chapter.

1.2.1.3 Non-classifiable Root Canal Configurations

Recent reports on the identification of external and internal anatomical canal variations using advanced 3D imaging technology have revealed that the morphological characteristics of the root canal system are highly complex, and many canal configurations have been described as ‘non-classifiable’ [31, 40–42]. It is obvious that the use of Roman numerals for describing the wide variations in the types of root canal configuration is impractical.

1.2.2 Introduction to the New Coding System for Root and Canal Morphology

Recently, an alternative coding system for classifying root and canal morphology was proposed, which provides detailed information on tooth notation, number of roots and root canal configuration [10]. The new system aims to provide a simple, accurate, and practical way for students/trainees, clinicians, and researchers to classify root and root canal configurations identified using any diagnostic method regardless of their accuracy and reliability.

1.2.2.1 Terminology

• – Root canal system:

The space within the tooth that contains pulp tissue. The root canal system is divided into two portions: the pulp chamber and the root canals.

• – Pulp chamber:

The portion of the pulp space within (or extending to just below) the anatomic crown of the tooth. In single-rooted teeth and double/multi-rooted teeth with middle or apical root bifurcations with a single canal coronally, it extends to the most apical portion of the cervical margin of the crown, and in double/multi-rooted teeth with coronal root and/or canal bifurcations (no single canal coronally), it extends to the floor of the pulp chamber located in the coronal third of the root. A chamber (accessory) canal is a small canal leaving the pulp chamber that (usually) communicates with the external surface of the root (including the furcation). It can be of any type (patent, blind, or loop).

• – Root canal orifice:

The opening of the canal system at the base of the pulp chamber where the root canal begins. Generally, it is located at or just apical to the cervical line.

• – Root canal configuration:

The course of the root canal system that begins at the orifice and ends at the canal terminus (minor apical diameter).

• – Major apical foramen:

The exit of the root canal onto the external root surface, which is normally located within 3 mm of the root apex.

• – Minor apical foramen/apical constriction:

The apical part of the root canal with the narrowest diameter which is generally 0.5–1.5 mm from the major apical foramen. It is the reference point often used as the apical termination of canal instrumentation and filling procedures.

• – Accessory canal:

A small canal leaving the root canal that (usually) communicates with the external surface of the root or furcation. Hence, it can be located anywhere along the length of the root (coronal, middle, or apical third) and can be any type (patent, blind, loop) (Figure 1.6). It also includes what have been in the past termed lateral canals. Apical delta (or apical ramifications) is the region at or near the root apex where the main canal divides into multiple accessory canals (more than two).

1.2.2.2 Classification

The new classification includes codes for three separate components: the tooth number, the number of roots, and the root canal configuration.

1.2.2.2.1 Tooth Number

The tooth number (TN) can be written using any numbering system (e.g. universal numbering system, Palmer notation numbering system, FDI World Dental Federation numbering system). If the tooth cannot be identified using one of the numbering systems (i.e. extracted teeth), then a suitable abbreviation can be used, for example UCI for upper (maxillary) central incisor (UCI).

1.2.2.2.2 Number of Roots

The number of roots (R) is added as a superscript before the tooth number (^RTN). For instance, ¹TN means that the tooth has one root. Any division of a root, whether in the coronal, middle, or apical third, will be coded as two or more roots. Accordingly, a bifurcation is represented as ²TN, and trifurcation is represented as ³TN and so on.

1.2.2.2.3  Root Canal Configuration

The type of root canal configuration (RCC) in each root is identified as a superscript number(s) after the tooth number and will define the continuous course of the root canal system starting from the orifice(s) (O), through the canal (C), and to the foramen (foramina) (F). The new system for root and canal morphology defines the root canal configuration with a start (root canal orifice) passing through the canal and ends at the apical foramen. Figures 1.7–1.9 show the application of the new system on different teeth with a range of root canal configurations. On some occasions, the root bifurcation in double/multi-rooted teeth is located in the middle or apical third, in which a common canal is present coronally that starts from the level of CEJ, similar to single-rooted teeth. This common canal is written as a superscript before describing the canal configuration for each of the roots (Figure 1.10). Recently, the new coding system has been refined for application in the primary dentition [43].

1.2.2.2.4 Location of Accessory Canals

The new coding system can also be used to classify accessory canals with canal configurations in a single code. The length of the root is divided into thirds (T): the coronal third (C), which starts from an imaginary line from the most apical portion of the pulp chamber, middle third (M), and apical third (A) ending at the canal terminus. Each third is identified as a superscript within parenthesis after the root canal configuration. In some instances, the accessory canal may not end in a foramen and in that situation, configuration code (1‐0) will describe a blind accessory, and code (2‐1‐0) will describe a looped accessory canal. Figures 1.11 and 1.12 show the application of the new coding system for accessory canals.

1.2.2.2.5 Presence of Dental Anomalies

The new coding system can be adapted for root anomalies by including codes for anomalies and their subtypes (if present). The abbreviation of the anomaly (A) is added between brackets. For example,

• (DE) refers to dens evaginatus affecting a given tooth. If more than one of the same anomaly exists in one tooth, then the number is written on the left of the anomaly. Thus, (2A) describes a tooth with two of the same anomaly; thus, (2DE) describes a tooth with two dens evaginatus.
• When the tooth has two or more different developmental anomalies, a comma (,) should be added between the initial letters of each anomaly (A1, A2). Thus, (DI, RD) describes a tooth with both a dens invaginatus (DI) and a root dilaceration (RD).
• A slash (/) should be used in fused teeth, for example, fusion of one tooth to a supernumerary tooth, or fused roots in double-rooted teeth such as C-shaped canals occurring in fused double-rooted mandibular molars [44]. Two slashes (//) should be used in fused teeth or roots with intercommunications in the root canal and/or pulp chamber.
• The subtype of each classified anomaly (if present) should be written as a superscript after the abbreviation of the anomaly. Thus, (DI^I) describes a tooth with a dens invaginatus type I [45, 46]. In some instances, it may be impossible to define a subtype of the anomaly during an examination (such as during conventional radiographic examination), or when it is not relevant within a specific clinical or experimental report; in such cases, writing the abbreviation of the anomaly without a subtype would be sufficient.

Figures 1.13 and 1.14 show the application of the new coding system in teeth with dental anomalies.

1.2.2.3 Applications of the New System in Teaching

To deliver the information required to allow dental students to learn and acquire knowledge for clinical practice is a significant responsibility [47]. Inadequate understanding and inability to systematically address normal and unusual anatomical variations of roots and root canals in a given tooth are the main causes of failure of primary root canal treatments as a consequence of persistent infection within the root canal space [48]. Root and canal morphology are integral components of the endodontic curriculum, and it is the initial educational step to develop understanding in tooth anatomy before practicing endodontic treatment and surgical procedures.

A recent national survey study compared feedback from undergraduate Malaysian dental students on both the Vertucci and Ahmed et al. systems [49]. The results revealed that 90% or more of students believed that the new system for classifying root and canal morphology was more accurate and more practical compared to the Vertucci classification and its supplemental configurations. More than 95% of students believed that the new system aided their understanding of root and canal morphology, and they would recommend its inclusion in preclinical and clinical courses (Figure 1.15). Similar results were observed amongst postgraduate dental students [49]. This is also consistent with results of another online survey undertaken amongst general dental practitioners (GDPs) and endodontists in Perú [50].

This favourable feedback is probably attributed to the fact that the new classification is an ‘open system’ that can describe the number of roots accurately and does not have certain types for categorizing the root canal morphology (no need to memorize certain categories). It is worth noting that the survey only focused on root canal configurations; however, participants in the survey undertaken in Malaysia raised comments on the possibilities of using the new system to classify accessory canals and anomalies [49]. Even though these anatomical landmarks were not included in that survey, such reflections demonstrate the ability of students to apply factual knowledge to understand, analyse, evaluate, and even create or add to the original product/system [51]. Similar survey studies on accessory canals and anomalies are needed, which have important clinical implications at the undergraduate and postgraduate level. Indeed, results of such surveys should not undermine the value of previous classification systems; students, GDPs, and endodontists still have to be aware of the advantages and limitations of Vertucci’s classification.

The canal coding system has recently been used with other teaching modules such as virtual reality [52].

1.2.2.4 Applications of the New System in Research

The new coding system has been used in a number of clinical and experimental CBCT studies on Egyptian [53], Chilean [54], Polish [55], Malaysian [56, 57] and South African [58] populations to describe root canal morphology in the anterior dentition, maxillary premolars and mandibular molars. For the anterior dentition, both Vertucci and Ahmed et al. systems were able to classify common configurations in maxillary and mandibular incisors; however, non-classifiable Vertucci configuration types were identified in mandibular incisors, which were classified using the new system [56]. The new system also was able to classify double-rooted canines in a more accurate manner compared to Vertucci’s classification, which does not consider the number of roots in the anterior dentition [56].

It has been reported that both systems were able to classify some canal configurations in single-rooted maxillary premolars (MP) similarly [53, 58]. However, confusions exist for Vertucci type IV for single- and double-rooted premolars, which are coded separately using the new system (codes ²MP B¹ P¹ and ¹MP²) (B: buccal, P: palatal) (Figure 1.16a), in addition to Vertucci type VIII (three separate canals), which has several presentations in the new system (such as codes ²MP B^1-2 P¹, which refers to double-rooted maxillary premolar with a canal configuration 1-2 in the buccal root and one canal in the palatal, and ³MP MB¹ DB¹ P¹, which refers to a three-rooted variant with three separate roots and one canal in each root [53, 58] (Figure 1.16b).

The literature is limited with regards to the application of the new system in molars [54]. One study used the new coding system to classify mandibular molars in a Chilean population using CBCT [54]. More studies are needed to provide evidence for its application in mandibular and maxillary molars in different population groups as well as to classify accessory canals and anomalies.

Based on evidence from the current literature, it appears that both the Vertucci and Ahmed et al. classifications are able to address simple root canal configurations similarly; however, the latter is able to address the number and location of roots in anterior and premolar teeth in addition to complex and non-classifiable Vertucci configurations in a more accurate manner [55, 56, 58]. Supplementary material is available online showing different applications of the new system in laboratory and clinical study models.

1.2.2.5 Applications of the New System in Clinical Practice

The application of the new coding system in clinical practice differs from research, the latter often being an observational analysis for anatomical features of specific teeth. In clinical practice, the tooth is subject to two phases – phase one is ‘observational’ where the operator usually undertakes a pre-operative 2D radiographic view (or CBCT) to study the initial morphological features of the tooth scheduled for treatment followed by an ‘intervention phase,’ which includes access cavity preparation, exploration, negotiation, troughing if required, instrumentation, and filling procedures – or surgery.

The interpretation of root canal morphology could vary through the phases. An example, based on the 2D pre-operative radiographic image in Figure 1.17, the operator would categorise this as a double-rooted tooth 36 in which the mesial root (M) appears to have two separate canals and the distal root (D) appears to have one canal configuration. Therefore, the initial code for this tooth is ²36 M² D¹ (Figure 1.17a). After root canal instrumentation and filling, two accessory canals were noted in the apical thirds of each of the mesial and distal root (Figure 1.17b). Therefore, the code was eventually ²36 M^2(A1) D^1(A1), which refers to double-rooted tooth 36 in which the mesial root has 2 separate canals and a single accessory canal in the apical third of the root, while the distal root has one canal and a single accessory canal in the apical third of the root. Another example is shown in Figure 1.17c,d. This means that the new coding system can be modified based on the operator’s interpretation along the treatment phases from diagnosis to root canal filling – or during surgery.

The interpretation of root canal morphology using the new coding system during the observational phase is important, especially for undergraduate and postgraduate students as well as GDPs where cases have to be pre-evaluated carefully to fit their level of knowledge and experience. This means a tooth code ²34 B^1-2 L¹ (double-rooted tooth 34 in which the buccal root is assumed to have canal configuration 1-2 and the lingual root has one canal) interpreted from a 2D pre-operative radiograph may not be suitable for an undergraduate dental student, and a tooth code (RD) ²36 M² D² (double-rooted tooth 36 in which both mesial and distal roots are dilacerated – RD – Root Dilaceration) may not be suitable for a GDP. Therefore, the new system can play a role in assessing case difficulty at the pre-operative stage, and also can provide a single code of the tooth after treatment, which may show other anatomical features, such as accessory canals.

1.2.2.6 Limitations and Technical Challenges

The assessment of apical canal configurations may vary depending on the method used for identification (experimental or clinical), which can be rather subjective amongst different observers [10]. For example, based on certain experimental measurements of canal dimensions or clinical negotiability, some apical bifurcations could either be classified as an apical delta/ramification (i.e. complex ramification of branches of the root canal located near, and open on, the root apex) or a division from the main canal (type 1-2). To date, a standard consistent view of such anatomy has not emerged, and therefore, the type of apical canal configuration should be classified based on the method and criteria used for its identification [10].

The interpretation of canal anatomy using the new coding system (as well as other classification systems) is highly dependent on the method used (i.e. staining and clearing, 2D radiographic, CBCT or micro-CT). Indeed, micro-CT is able to show the fine details including delicate canal branching and accessory canals, which cannot be seen in CBCT images; therefore, the coding of the same tooth using both techniques will be different.

1.3 Advances in Apical Canal Morphology

Anatomical challenges and subsequent procedural errors in the apical third of root canals are associated with less favourable treatment outcomes compared with those that occur in the coronal third [59]. Therefore, the apical anatomy should be evaluated and understood by clinicians prior to treatment.

The anatomical landmarks of the apical root canal have been investigated since the beginning of the twentieth century. The development of new imaging systems has generated substantial data to increase knowledge regarding the morphological characteristics of the apical region of the root and root canal, such as the cemento-dentinal junction (CDJ), apical constriction (AC), apical foramen (AF), isthmuses, accessory canals (ACCs), and bifid root apices (Figure 1.18).

1.3.1 Cemento-dentinal Junction (CDJ)

The CDJ is a structure that can be observed in histological sections; however, clinically and radiographically, locating the CDJ is impossible. The cementum extends into root canals by covering dentine and the CDJ is located at various levels around the circumference of the canal wall in the region of the apical foramen. It is located at different levels within canals in different tooth types. Usually, the CDJ is not in the same position as the AC, but due to the deposition of cementum with age, the rate of coexistence at the same point as the AC increases [60]. There are no odontoblasts in the most apical section of the pulp, where cellular cementum lies inside the canal [60, 61]. Although the nature of the tissue found apically is of academic interest, it has no clinical impact [61].

1.3.2 Apical Constriction (AC)

The AC, also termed the physiological foramen, minor diameter, and minor apical foramen, is defined as the narrowest diameter of the root canal towards the root apex [30, 62] (Figure 1.19). However, it is not always present [63–66]. According to many, it is the most suitable apical reference point within the canal for clinicians to complete canal shaping, cleaning, and filling. Biologically and logically, resection of pulp tissue at this narrowest point causes less inflammation [67]. Additionally, during root canal procedures, this narrowest point is thought to reduce the potential undesirable effects of endodontic procedures or materials on periapical tissues [67, 68].

These reasons make the AC clinically relevant. Several studies have examined the location of AC and its topography using various techniques [62, 65, 69, 70]. However, the absence of a standard for studies that aimed to reveal the presence and location of the AC, along with the different perspectives among researchers, has led to different results regarding its existence. To date, the smallest diameter in longitudinal sections of the apical canal or the smallest area of apical canal in horizontal sections was used to detect the AC. In some studies using longitudinal sections, the parallel and flared types of apical canal were considered to have no constriction [63–65], whereas the AC can be detected as the narrowest cross-sectional area of the canal in parallel and flared types of apical canal [71].

The bulk of information on the position and shape of the AC is based on inspecting and measuring the smallest diameter in longitudinal sections of apical root canals. The longitudinal sectioning method is sensitive and vulnerable to procedural errors; detection of the direction of the longitudinal section has no standard methodology or criteria for defining the landmarks, and excess removal of dentine may result in inaccurate measurements [72]. In addition, the individual operator is responsible for carefully detecting and measuring the narrowest point of the canal. Thus, this method is greatly influenced by their experience. Also, ovality, complexity, and multiplanar curvatures of root canals pose major challenges for the accurate investigation of the AC in longitudinal sections. Clearly, the section must be at the centre of the canal throughout the canal axis to allow accurate measurements.

Currently, micro-CT technology is considered the most accurate research tool used to study root canal anatomy [73]. This non-invasive, non-destructive, high-resolution technology allows the 2D and 3D study of root canal anatomy. The apical root canal anatomy can be investigated from various angles, both in 2D and 3D, together or separately. Also, quantitative and qualitative measurements can be performed using micro-CT [73].

Micro-CT has the potential to overcome most limitations inherent in other techniques used to investigate the AC. Several studies have used micro-CT to examine the AC [63–65, 71, 72, 74]; however, most studies examined longitudinal sections of the apical root to detect the location and the topography of the AC. Unfortunately, similar to previous studies using conventional sectioning techniques, the results of these studies have many variables and incomparable results on the AC. It is obvious that the longitudinal sectioning method, even if a technological device such as the micro-CT is used, is not suitable to detect the smallest diameter of root canals. The topography and the location of the minor diameter vary from one longitudinal section to the other even though the interval between the sections is in microns [63, 72] (Figure 1.20).

In addition, the plane of the smallest diameter of the root canal may be in different directions from the coronal to the apical. It is not possible to measure the smallest diameter of the root canal at different levels only in one longitudinal section (Figure 1.21). Moreover, as root canals are not completely round, the smallest diameter displayed in the longitudinal section does not necessarily correspond to the narrowest area of the canal (Figure 1.21). Logically, measuring ‘the smallest area’ is more suitable to detect a constriction than measuring ‘the smallest diameter’ of the canal. Contrary to longitudinal sections, the smallest cross-sectional area of root canals can be measured in horizontal sections (Figure 1.21). The study of the AC, based on horizontal sections, has been the subject of several micro-CT studies [71, 72, 74, 75]. On average, the AC – the narrowest area of the root canal – is positioned 0.1–0.4 mm coronal to the major apical foramen (MAF) [71].

1.3.3 Major Apical Foramen (MAF)

From the AC, the canal approaches the MAF, also termed the major apical diameter, and exits on the outer surface of the root [30, 76]. Although historically it has generally been accepted that the distance from the AF to the anatomic apex (the tip of the root as determined morphologically) is 0.5 mm, the location and topography of these anatomic landmarks are highly variable and related to tooth type and age; because of the formation of cementum, the distance is greater in older individuals [60, 77].

The MAF is rarely located at the anatomic apex of a root and is more often found on the mesial, distal, buccal, or lingual/palatal surfaces. Deviations of the location of the MAF occur not only in a transverse direction but also in the vertical direction [69]. Moreover, the AFs of different root canals in the same root are likely to be located at different levels of the root (Figure 1.22). In the mesial roots of the mandibular first molar, the distances from the AF of the mesiobuccal and mesiolingual canals to the anatomical apex can be more than 3 mm, and the deviation between the location of the AF of these canals may approach 2.5 mm [78]. The greatest deviation values were found in the location of the AF of middle mesial root canals of mandibular molars [78, 79], which might be located 3.3–6.2 mm from the root apex [79]. The detection of the deviation of the AF (using electronic foramen locators) is important to avoid mishaps during preparation such as incomplete or over-preparation and filling of the canal.

In recent years, the number of studies examining the entire apical root canal has increased. In these studies, 2D measurements such as area, perimeter, minor and major diameters, form factor, and roundness of the apical root canal have been measured at 0.5 mm or 1 mm intervals. In addition, surface area, structure model index (SMI) and volume of the canal have been measured independent of the operator’s experience as 3D measurements (Table 1.1) (Figure 1.23). These detailed and precise 2D and 3D measurements from the apical root canal third are expected to guide clinical procedures.

The original diameters of the apical third of the mesial canals of mandibular molars (MM) [²MM M² D^O-C-F (Vertucci type IV)] have been matched with the dimensions of files to determine the instrument sizes/tapers that would have a greater chance of touching all the canal walls in the apical third [80]. According to the results of this study, size 40, 0.06 taper and size 45, 0.02 taper files had the best results in the apical third and would have more chances of contacting a greater area of walls. However, it was also reported that in the apical third the canal would not be completely prepared by any currently available instrument system in 78% of teeth. Similarly, the major diameters of the apical third of mesial canals of mandibular molars [²MM M^2-1 D^O-C-F (Vertucci type II)] have been reported to be larger than the size of the root canal files that can be used in mesial root canals [14]. Although the area of touched root canal walls can be increased using larger files, the risk of lateral perforation and apical transportation should be considered. Literature revealed the presence of apical root canals with various dimensions in different root canal configurations, which should be investigated in further studies.

1.3.4 Isthmus

An isthmus is defined as a narrow communication between two or more canals in a root [30]. The length and types of isthmus can vary and they may occur at any level of the root. To date, various classifications based on 2D and 3D images and terminology have been used to define an isthmus. Hsu and Kim [82] classified five types of isthmus that might be encountered on resected root surfaces during apical surgery (Table 1.2). This 2D-based classification system categorises various types of isthmuses at different levels of the same root and, therefore, does not provide a complete and accurate description as it does not take account of the distance of the resected surface from the apex. This discredits the different categories in this 2D classification as the views of the resected root surface provide detailed information on the nature of isthmus and its relative position to root canals. Indeed, this classification is valid for apical surgery as clinicians can use it while observing a resected root surface with a micro-mirror after resecting the apical part of the root. However, the system does not include any 3D information on the isthmus and thus has its limitations.

Because many morphological features of the isthmus observed in 3D do not fit the 2D classification, other terms and classification systems have been used to define an isthmus in 3D [14, 15, 81, 83]. These studies reported a high frequency of isthmuses (up to 85%) at the apical 5 mm. Mannocci and colleagues [83] investigated the isthmus using micro-CT and counted the number of transverse sections with a definite connection between the canals. Fan and colleagues [81] classified four types of isthmus based on 2D and 3D morphologic features. In this classification, the coronal and apical extent of an isthmus was determined in 2D cross-sections, and the types of isthmus were determined in 3D reconstructed models (Table 1.3). Detecting the borders of an isthmus without a definite connection between the two root canals is highly subjective, especially in long oval root canals, where detecting a definite border and distinguishing the isthmus is not possible. The isthmus is an anatomical structure, and defining its borders is necessary. The isthmus roof and isthmus floor have been defined in recent studies [14, 15]. The most coronal level of the isthmus, where the definite connection between two root canals occurs, is named the isthmus roof, and the most apical level of this definite connection is named the isthmus floor. By using these borders, it is possible to measure and compare the volume, length, and surface areas of the isthmus (Figure 1.24).

Keleş and Keskin [14] examined the effects of an isthmus on the apical canal anatomy in mandibular first molars of code ²MM M^2-1 D^O-C-F (Vertucci type II) and reported that when the isthmus length and the distance from the AF to the isthmus roof decreased, the major diameter of the apical root canals increased at any level within the apical third. In addition, this study revealed that the major diameter of the apical root canal at any level within the apical region is larger than the size of root canal files normally used as the master apical file in the mesial roots of mandibular molars (Figure 1.25).

Keleş and Keskin [15] defined a ‘band shaped isthmus,’ which has an isthmus roof and isthmus floor. In order to have an isthmus floor, an isthmus must have at least two apical root canals. The band shaped isthmus has a long oval cross section, and the isthmus floor is located in close proximity to the AF. It was reported that the length of apical root canals up to the isthmus floor was in the range 1–1.5 mm (Figure 1.26). The long oval cross-sectional structure of band-shaped isthmuses is a challenge for root canal treatment procedures. It was reported that canal filling techniques resulted in void percentages in the range of 13–15% in the band-shaped isthmuses [84]. Non-filled band-shaped isthmus areas with proximity near to the apical foramina could be seen as a potential risk factor for post-treatment disease.

Although an isthmus is one of the most difficult challenges for root canal and surgical procedures, in terms of preparation, disinfection, and filling of the apical root canal system, the accurate identification of an isthmus and its features, such as length and type, is impossible clinically despite recent technological advances in imaging [85]. Moreover, although an isthmus requires special consideration in all procedures of root canal treatment, they are often inaccessible to root canal instruments and irrigants. In addition, as a side effect, dentine debris and pulp tissue can be packed into an isthmus during canal preparation [86] (Figure 1.27). Although additional irrigation protocols are advised to remove debris from an isthmus, no irrigation protocol can result in a totally clean, debris-free isthmus [87]. Remaining debris may contain microorganisms, which might both endanger the outcome of treatment and physically act as a barrier against root canal filling materials and irrigants [87].

1.3.5 Accessory Canals and Apical Deltas

ACCs are another anatomic structure to be considered in the apical third of roots. ACCs are any branch of the main pulp canal that usually communicates with the external surface of the root via accessory foramina [30] (Figure 1.28). An apical delta contains multiple accessory canals (more than two) that branch out from the main canal near the root apex [30]. The term ramification includes furcation, lateral and apical accessory canals, as well as any unusual intracanal anatomy [30]. The variation in the morphology of ACCs in the apical third has resulted in various terminology and classification being used (Table 1.4